Demand for electrochemical elements, such as small-sized, lightweight, lithium ion batteries with high energy density that are free of memory effects and tolerable to repetitive charging/discharging, or electric double layer capacitors with excellent charge/discharge cycle life, has rapidly expanded, in order to take advantage of their qualities. Since lithium ion rechargeable batteries have a relatively large energy density, and are small-sized and lightweight, they have been utilized in the fields of cell phones, notebook personal computers, etc. Also, since electric double layer capacitors are tolerable to rapid charging/discharging, and have excellent charge/discharge cycle life, they have been utilized as memory backup small-sized power supplies for PCs, etc. Furthermore, applications of these electrochemical elements as large power supplies for electric automobiles and electric storage facilities have been expected. As described above, with recent achievements of multifunction concerning purposes such as for electronic equipment, communication equipment or automobiles, the electrochemical elements have been expected to achieve further improvements of properties, such as even higher output, higher capacity, or improvements of mechanical properties. In such a situation, various improvements have been made in methods for forming an electrode for electrochemical elements, in order to improve performance of electrochemical elements.
For example, an electrode for electrochemical elements can be obtained in the following manner: as a coating film material, an electrode material containing an electrode active material and the like is formed into a sheet, and then, the sheet (electrode composition layer) is press-bonded to a collector that is a coating object, thereby obtaining an electrode. In order to allow an electrode for electrochemical elements to have higher capacity, it was required that the electrode active material be formed so as to have a high density.
In order to obtain such a high-density sheet-shaped molded product, JP-A-2001-230158 describes a method in which a material including carbon fines, a conductive assistant and a binder is mixed and kneaded to produce a kneaded product, and then, the kneaded product is formed into a sheet-shaped molded product with a predetermined thickness by use of a roll press. Specifically, the method of forming a sheet-shaped molded product according to JP-A-2001-230158 utilizes a production apparatus that retains the kneaded product in a space formed by a pair of rolls and a partition plate and that conducts press-molding of the kneaded product with the pair of rolls to obtain the sheet-shaped molded product. This improves the density of carbon fines, and makes it possible to achieve the high capacity.
Furthermore, in the specification of Japanese Patent No. 4840358, a method is proposed, in which an electrode composition layer including composite particles in which aggregates of an active material or conductive material having a relatively large particle diameter are covered with an active material or conductive material having a relatively small particle diameter is press-bonded to a collector to produce an electrode sheet for electrochemical elements in a state where the shapes of composite particles remain unchanged.